1. Field of the Invention
The present invention relates to a coating composition for a heat sealable, inkjet printable image transfer material, an article comprising said coating composition in form of at least one heat-sealable, inkjet printable layer, a method for applying graphical information to a surface of an object using a heat-sealable, inkjet printable image transfer material comprising said coating composition and a method of making such a coating composition.
2. Description of the Related Art
Inkjet printing is a widely used process for applying graphical information and images, including multi-color images, to a substrate. In such a process, solvent-containing ink droplets are deposited from a nozzle onto an absorbent substrate, e.g. paper, to form the image. Rapid absorption of the ink into the substrate is a requirement for good image quality and to obtain images with a high color gamut and without printing defects. Most substrates need a special inkjet ink absorbing coating for good printing results. The inkjet coating formulations have to be adjusted to the specific substrate, papers for instance need different coatings than films. Moreover, there are numerous substrates which cannot be imaged in good quality by direct inkjet printing, including, for instance, substrates made of glass, metals, plastics and the like.
For this reason, image transfer materials have been developed comprising a film of readily printable material. The ink receiving film of the image transfer material usually is first printed and then transferred and adhered by means of one or more suitable adhesive layers to various substrates, including non-absorbent, sensitive, very thin or very thick materials and/or those having uneven surfaces, which cannot be readily provided with graphical information or an image by direct inkjet printing.
In order to provide high quality images which are applicable to a wide range of materials, an image transfer material should be transparent and substantially colorless, so that after transfer to a particular substrate essentially only the information/image provided by said material is perceived by an observer, but no unprinted parts of the transfer material. Ideally, such a transfer material is capable of adhering to a wide range of different substrate materials, including materials made of metal, glass, wood, plastics, ceramics, elastomers, rubber, fabrics, textiles, paper, cardboard and the like as well as mixtures thereof, without being limited to these.
WO 2008/002795 A1 describes image transfer films for graphic substrates, which can be laminated onto a graphic substrate by means of heat and pressure. In the method described therein, an image is printed onto a transparent thermoplastic protective layer by an inkjet printer using an organic solvent-based ink comprising said solvent, a thermoplastic material and a pigment. Preferably, the transparent thermoplastic protective layer and the thermoplastic material of the ink both comprise an urethane-acrylate copolymer and soften during lamination, thereby adhering or bonding to the graphic substrate. As graphic substrates, mainly polymeric materials are discussed in WO 2008/002795 A1.
Further heat transfer materials, which may be applied to a broader variety of substrates, usually include a separate adhesive film coated or laminated to one side of the ink receiving layer of the image transfer material. Such articles are described, for instance, in WO 2009/018419 A1. While such materials usually can be applied to a wide variety of different substrates, including those having uneven surfaces, the need of a separate adhesive layer is a drawback in producing such films.
Moreover, depending on the thickness and the kind of layers used in such a multi-layer coating stack, the transparency of the image transfer material may drop and delamination within the layered structure may occur under certain conditions.
Thus, it would be advantageous to provide an inkjet printable image transfer material which combines the inkjet receiving and the adhesive function in one single layer, while nevertheless being applicable to substrates comprising a variety of different materials and/or structures, including curved and/or uneven surfaces. Further, unless intentionally modified by adding a colorant, the non-printed parts of such an image transfer material should be essentially colorless and transparent. Moreover, it should be possible to provide a very thin transfer image using said material.
It has surprisingly been found that such an inkjet printable image transfer material which is heat-sealable can be provided when a mixture of at least one cellulose-based binding agent, at least one rosin-based tackifier and at least one plasticizer is used. Using said combination, it is possible to provide a coating, which combines adhesive and inkjet-receiving properties in one single layer, so that printable and transferable films can be provided which are chemically and mechanically durable, even if provided at a rather low thickness. Using said combination of ingredients, it is further possible to provide a substantially colorless and transparent inkjet printable image transfer material which can be applied to a variety of different materials, including glass, plastics, paper, metal and the like, without being limited to these, at comparable low transfer temperatures. Herein, a transparent material preferably has a light transmittance of at least 60%, determined according to DIN 53147 or ISO 22891. Moreover, the outer surface of the transferred image material can be modified after printing to adjust its surface structure and/or gloss to match the look and feel of the target material.